Now you can sign up to receive alerts from accounts you follow. Not every tweet will be pushed to your phone, only ones the account marks as alerts. You don't have to have Twitter's iOS or Android apps to get the alerts, but if you do, they'll be sent as push notifications. If you don't have the apps, you'll get alerts as SMS texts. If you're scrolling through your stream, alert tweets will be marked with an orange bell.

Alerts are not designed for important life updates from friends. Twitter said in a Wednesday blog post that the system is limited to law enforcement and public safety agencies; emergency management agencies; city and municipal governments and their representatives; county and regional agencies; and state, federal, and national organizations, like the Federal Emergency Management Agency. A complete list of more than 100 participating agencies is available here. Organizations have to enroll in Twitter's alert program to send out alerts.

The program is opt-in, so you won't be automatically signed up for an account's alerts just because you follow them. Sign up for an account's alerts by going to twitter.com/username/alerts. Replace username with the account name. Follow the directions to subscribe to alerts, as seen on the right.

For better or worse, people turn to Twitter during times of crisis to find out information (see: Hurricane Sandy, the Boston Marathon bombings). Last year, Twitter began piloting an emergency alert system in Japan called Lifeline in partnership with the Prime Minister's Lifeline Commission. Twitter worked with local governments in Japan to find important Twitter accounts, or ones people would turn to in a crisis, and categorized them by postal code. Japanese Twitter users can access Lifeline accounts by searching for their postal code on twitter.com.

Twitter's new alerts are not quite the same as Lifeline, but could be just as useful during emergencies. The State Department's @TravelGov account plans to use Twitter's alerts to let people know when it's unsafe to travel to a country due to civil unrest or natural disasters. The @BostonPolice account tweeted information and safety alerts throughout the Boston Marathon bombing aftermath. People are already turning to Twitter for this information--now it's push instead of pull.

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